Field of the Invention
The invention is related to a structure for manufacturing and using a fabrication technique of micro-electronics so as to include an optically black surface adapted to function as an absorber or emitter over a predetermined wavelength range.
An optically black surface can be used as an absorber of a limited wavelength range in detectors of optical radiation and an emitting surface in thermal emitters of the optical wavelength range. Particularly in infrared detectors of the bolometer and thermopile type, there is a need for a surface capable of efficiently absorbing radiation over a wide optical wavelength spectrum. Correspondingly, infrared radiation emitters require a surface with a high emissivity. The principle of reciprocity says that a good absorber also is a good emitter, thus making the same surface suitable for use in both applications.
In the context of the present invention, a black surface functioning over a predetermined wavelength range must be understood to refer to a surface of high absorptance over said wavelength range. Respectively, a white surface functioning over a predetermined wavelength range must be understood to refer to a surface of high reflectance over said wavelength range. When such an optically black surface is desired to be used in a detector functioning over said wavelength range, the ideal surface of the detector behaves as a black surface over said wavelength range and as a white or transparent surface outside said wavelength range. Thus, wavelengths falling outside said wavelength range cannot disturb the measurement to be performed.
In conventional techniques, the manufacture of broadband absorbers has been carried out using polymers. This kind of absorber has a thin polymer film deposited on, e.g., a bismuth layer. Additionally, the base polymer could have been blended with absorption-improving agents such as carbon black particles. While polymer-based absorbers have been easy and economical to manufacture, they have also been hampered by a number of drawbacks. The polymers used as the absorber layers have been sensitive to their operating environment, particularly to moisture, and the performance of polymer absorbers have remained far from perfect. Furthermore, the thermal mass of detectors has been relatively large, thus making detectors of the polymer-based absorber type relatively slow by their response speed. An additional disadvantage of polymer films has been their poor high-temperature performance, which excludes their use as an emitting surface in heatable IR emitters.
Also absorber and emitter components are known based on semiconductor technology. In a paper written by K. C. Liddiard in the publication Infrared Physics, 1993, Vol. 34, 4, p. 379 ff., is described a multilayer film structure in which the uppermost layer is a semitransparent metallic thin film is provided with, thereunder a lossless dielectric layer and a lowermost metallic thin film acting as an infrared mirror. The multilayer structure is grown on an unthinned glass substrate. The basic disadvantage of this structure is its slow response and low sensitivity, both resulting from its relatively large thermal mass. The structure is further characterized by a substantially high loss of heat by conduction into the substrate. The semitransparent metallic thin film is difficult to produce to a correct thickness, and moreover, is readily destroyed when serving as the outer surface of the detector.
In a paper published by L. Dobrzanski et al. in the publication Proc. Euro-sensors X, 1996, Lowen, p. 1433 ff., is described a structure further developed from the above-described type by having the absorber deposited on a 100-200 .mu.m thick silicon wafer. In the structure, there is first deposited on a silicon wafer a 0.2-1.5 .mu.m thick, lossless film of silicon nitride, and next thereon, a 0.1-1.5 .mu.m thick, lossy film of doped polycrystalline silicon. The reason for selecting polycrystalline silicon as the top layer material is because of its good performance at elevated temperatures and relatively high temperature coefficient of resistivity. Under the silicon and silicon nitride layers is produced an infrared-reflective mirror by sputtering a layer of tungsten or a nickel-chromium alloy from below via openings made into the substrate.
The above-described structure has a number of disadvantages. The bottomside metallization layer of the component permits high lateral conductivity of heat into the substrate. Since the metallization layer has no protective film thereon, the structure is also unsuitable for use in thermal emitters. Moreover, simultaneous thermal and optical optimization of layer thicknesses in the structure is impossible.